We are studying the concentration and patterns of secretion of gonadal hormones during the annual reproductive cycles of lizards, crocodilians, and turtles by steroid radioimmunoassay, and how these changes relate to naturally occurring species-typical reproductive behavior patterns. Experiments to determine the hormonal control of male and female reproductive behavior in reptiles are also being conducted. Direct versus indirect action of androgens on central and peripheral tissues in reptiles are being examined by behavioral testing and histological examination of the renal sex segment and oviduct in response to exogenous steroids and their antagonists. Steroid-sensitive sites in the male and female brain of lizards have been identified by autoradiography and lesion and intracranial implantation of steroid hormones are among the techniques being used to investigate the role of these areas in the regulation of social behavior.